WordNet is a powerful lexical  reference system that combines aspects of
dictionaries  and thesauri  with  current  psycholinguistic theories  of
human lexical memory. It is produced by the Cognitive Science Laboratory
at Princeton University, under the direction of Professor George Miller.

In WordNet, words  are defined and grouped into various  related sets of
synonyms.  Not only  is the  system  valuable to  the casual  user as  a
powerful thesaurus and dictionary, but also  to the researcher as one of
the few freely available, lexical databases. WordNet is available via an
on-line interface and also as easy-to-compile C source code for Unix.

WWW: http://wordnet.princeton.edu/